The studies are part of an overall attempt to identify, localize, and characterize possible neurotransmitters in the various parts of the crustacean stomatogastric nervous system (stomatogastric ganglion and its associated afferent and efferent nerve trunks and muscles, esophageal ganglion, connecting nerves from the brain, and commissural ganglia). The problem is to locate the neuron somata producing identified biogenic amines and to ascertain the possible site(s) of their terminations. 1) Work on biogenic amines will include: a) localization of catecholamines and serotonin by fluorescence histochemistry (Falck method); b) concurrent electron microscopy of similar specimens to describe the location of dense-core vesicles (depletion of the amines by reserpine will be employed as one type of control prior to both histochemical and ultrastructural studies); c) chemical analyses of biogenic amines in the same tissues. 2) Another aspect of this work involves electrophysiological study of the neuromuscular junctions between the axonal branches of an identified neuron in the stomatogastric ganglion and the particular stomach muscle which it innervates. This includes examination of reversal potentials; study of ion flows underlying the conductance changes in the muscle fibers; iontophoresis of suspected transmitters and comparison of the post-synaptic response obtained with that resulting from electrical stimulation of the nerve to this muscle.